A procedure that sometimes helpt me to improve my extracted protein for ELISA is to switch the extractionbuffer to the dilutionbuffer by ultrafiltration.
This has also the avantage that you will get rid of the smaller proteins that can interact in the ELISA.

-Gerard-

I agree reagarding the ultrafiltration or dialysis...especially if your extraction buffer has surfactants! Although I am unfamilir with plant tissue assays if your assay has sufficient analytical range and you don't need too much sensitivity at the low end running samples diluted 1:10 will help reduce any matrix interference (of course correct for the dilution factor by X 10 for result).